removing expand_user from config.py

- not needed since these variables will be referring to S3
making S3 cache code check for optional boto dependency

- removing boot module from requirements
- adding documentation to say that boto module is required to use
  S3 cache option
moved S3 cache methods to __init__.py

- so they can be used for sdists if needed
- removed boot imports from top of files
- inlined several helper methods so the work with the local import of boot