Added readonly flag for s3 cache

# Purpose
The purpose of this is to allow CI to keep the s3 cache current and developers will be able to prime their cache from the s3 source.

# Tests
Could not see any existing tests for the Cache s3 cache. Here are my manual tests.

### Intial with PIP_ACCEL_S3_READONLY=1
```
2014-11-20 21:34:27 leigh-XPS13 pip_accel[11941] INFO Unpacking source distribution(s) ..
2014-11-20 21:34:27 leigh-XPS13 pip_accel[11941] INFO Executing command: pip install --no-install --no-index --find-links=file:///home/leigh/.pip-accel/sources --build-directory=/tmp/tmpN8Oqrg scriptine
Ignoring indexes: https://pypi.python.org/simple/
Downloading/unpacking scriptine
  Running setup.py egg_info for package scriptine
    
    warning: no previously-included files matching '*.pyc' found under directory 'test'
Successfully downloaded scriptine
2014-11-20 21:34:27 leigh-XPS13 pip_accel[11941] INFO Finished unpacking 1 source distribution in 0.21 seconds.
2014-11-20 21:34:27 leigh-XPS13 pip_accel[11941] INFO Installing from binary distributions ..
2014-11-20 21:34:28 leigh-XPS13 pip_accel.caches.s3[11941] INFO Checking if distribution archive is available in S3 bucket: Linux-x86_643/v7/scriptine:0.2.0:CPython-2.7.tar.gz
2014-11-20 21:34:28 leigh-XPS13 pip_accel.bdist[11941] INFO Building scriptine (0.2.0) binary distribution ..
2014-11-20 21:34:29 leigh-XPS13 pip_accel.bdist[11941] INFO Finished building scriptine (0.2.0) in 0.41 seconds.
2014-11-20 21:34:29 leigh-XPS13 pip_accel.caches.s3[11941] INFO Skipping S3 Upload, S3 Readonly enabled
2014-11-20 21:34:29 leigh-XPS13 pip_accel[11941] INFO Finished installing 1 requirement in 1.29 second.
```

### Second Clean Run without PIP_ACCEL_S3_READONLY set
```
2014-11-20 21:34:43 leigh-XPS13 pip_accel[11974] INFO Unpacking source distribution(s) ..
2014-11-20 21:34:43 leigh-XPS13 pip_accel[11974] INFO Executing command: pip install --no-install --no-index --find-links=file:///home/leigh/.pip-accel/sources --build-directory=/tmp/tmpc66UES scriptine
Ignoring indexes: https://pypi.python.org/simple/
Downloading/unpacking scriptine
  Running setup.py egg_info for package scriptine
    
    warning: no previously-included files matching '*.pyc' found under directory 'test'
Successfully downloaded scriptine
2014-11-20 21:34:43 leigh-XPS13 pip_accel[11974] INFO Finished unpacking 1 source distribution in 0.19 seconds.
2014-11-20 21:34:43 leigh-XPS13 pip_accel[11974] INFO Installing from binary distributions ..
2014-11-20 21:34:43 leigh-XPS13 pip_accel.caches.s3[11974] INFO Checking if distribution archive is available in S3 bucket: Linux-x86_643/v7/scriptine:0.2.0:CPython-2.7.tar.gz
2014-11-20 21:34:44 leigh-XPS13 pip_accel.bdist[11974] INFO Building scriptine (0.2.0) binary distribution ..
2014-11-20 21:34:44 leigh-XPS13 pip_accel.bdist[11974] INFO Finished building scriptine (0.2.0) in 0.41 seconds.
2014-11-20 21:34:44 leigh-XPS13 pip_accel.caches.s3[11974] INFO Uploading distribution archive to S3 bucket: Linux-x86_643/v7/scriptine:0.2.0:CPython-2.7.tar.gz
2014-11-20 21:34:44 leigh-XPS13 pip_accel.caches.s3[11974] INFO Finished uploading distribution archive to S3 bucket in 0.3 seconds.
2014-11-20 21:34:44 leigh-XPS13 pip_accel[11974] INFO Finished installing 1 requirement in 1.68 second.
```

### Third Clean run with PIP_ACCEL_S3_READONLY set and bucket warmed
```
2014-11-20 21:34:58 leigh-XPS13 pip_accel[12005] INFO Unpacking source distribution(s) ..
2014-11-20 21:34:58 leigh-XPS13 pip_accel[12005] INFO Executing command: pip install --no-install --no-index --find-links=file:///home/leigh/.pip-accel/sources --build-directory=/tmp/tmpgIpc12 scriptine
Ignoring indexes: https://pypi.python.org/simple/
Downloading/unpacking scriptine
  Running setup.py egg_info for package scriptine
    
    warning: no previously-included files matching '*.pyc' found under directory 'test'
Successfully downloaded scriptine
2014-11-20 21:34:58 leigh-XPS13 pip_accel[12005] INFO Finished unpacking 1 source distribution in 0.25 seconds.
2014-11-20 21:34:58 leigh-XPS13 pip_accel[12005] INFO Installing from binary distributions ..
2014-11-20 21:34:59 leigh-XPS13 pip_accel.caches.s3[12005] INFO Checking if distribution archive is available in S3 bucket: Linux-x86_643/v7/scriptine:0.2.0:CPython-2.7.tar.gz
2014-11-20 21:34:59 leigh-XPS13 pip_accel.caches.s3[12005] INFO Downloading distribution archive from S3 bucket ..
2014-11-20 21:35:00 leigh-XPS13 pip_accel[12005] INFO Finished installing 1 requirement in 1.37 second.
```

### Forth clean run without PIP_ACCEL_S3_READONLY set and s3 cache warmed
```
Uninstalling scriptine:
  Successfully uninstalled scriptine
(venv)(s3_readonly)leigh@leigh-XPS13:~/git/pip-accel$ PIP_ACCEL_S3_BUCKET=pip-accel.immun.io PIP_ACCEL_S3_PREFIX="`uname -s`-`uname -m`3"pip-accel install scriptine
install: missing destination file operand after ‘scriptine’
Try 'install --help' for more information.
(venv)(s3_readonly)leigh@leigh-XPS13:~/git/pip-accel$ PIP_ACCEL_S3_BUCKET=pip-accel.immun.io PIP_ACCEL_S3_PREFIX="`uname -s`-`uname -m`3" pip-accel install scriptine
2014-11-20 21:35:18 leigh-XPS13 pip_accel[12045] INFO Unpacking source distribution(s) ..
2014-11-20 21:35:18 leigh-XPS13 pip_accel[12045] INFO Executing command: pip install --no-install --no-index --find-links=file:///home/leigh/.pip-accel/sources --build-directory=/tmp/tmpHQ4qv4 scriptine
Ignoring indexes: https://pypi.python.org/simple/
Downloading/unpacking scriptine
  Running setup.py egg_info for package scriptine
    
    warning: no previously-included files matching '*.pyc' found under directory 'test'
Successfully downloaded scriptine
2014-11-20 21:35:18 leigh-XPS13 pip_accel[12045] INFO Finished unpacking 1 source distribution in 0.19 seconds.
2014-11-20 21:35:18 leigh-XPS13 pip_accel[12045] INFO Installing from binary distributions ..
2014-11-20 21:35:19 leigh-XPS13 pip_accel.caches.s3[12045] INFO Checking if distribution archive is available in S3 bucket: Linux-x86_643/v7/scriptine:0.2.0:CPython-2.7.tar.gz
2014-11-20 21:35:19 leigh-XPS13 pip_accel.caches.s3[12045] INFO Downloading distribution archive from S3 bucket ..
2014-11-20 21:35:20 leigh-XPS13 pip_accel[12045] INFO Finished installing 1 requirement in 1.08 second.
```