Move back to gcc4.7.1

Summary: Dhruba can't compile on gcc4.8.1 so I'm moving temporarily back to 4.7.1 until we figure out what's wrong with 4.8. on his server.

Test Plan: It can compile on my devserver, but please 'arc patch' this diff and try compiling on your machine

Reviewers: dhruba, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14139
Add License message to public header files.

Summary:
Add License message to public header files.

Test Plan:

Reviewers:

CC:

Task ID: #

Blame Rev:
Add a compilation guide to rocksdb

Summary: as title.

Test Plan: no

Reviewers: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14121
Move the compiler back to 4.8.1 + more small fixes

Summary:
1. Moved the compiler back to 4.8.1 and uses Centos 5.2 binaries if OS is Centos 5.2.

2. Fixes this issue: https://github.com/facebook/rocksdb/issues/7

3. We use lot of c++11 features, so we can't pretend we can compile without them. Makes it a first class dependency.

4. Fix blob_store_test, which failes on Ubuntu with "too many files opened" error

5. Removed dependency on port/port_chromium.h, which does not even exist on our system

Test Plan: make clean; make check

Reviewers: dhruba, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14145
Compilation.md

Summary: Explained what it takes to compile it on linux.

Test Plan: -

Reviewers: dhruba, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14151