slightly clarified the "license" portion of `README.md" + formatting all over

There are commercial open source applications, hence proprietary seems to be a better term.

Also:
 - Trimmed header `=`'s to match text length
 - made libgit2 stand out everywhere
 - pretty printing links
 - irc link for those with the correct setup
slightly clarified the "license" portion of `README.md" + formatting all over

added the second change suggested by @nulltoken (with two extra words :) )
slightly clarified the "license" portion of `README.md" + formatting all over

fixed typo `can can` -> `can`
Merge pull request #1900 from haneefmubarak/patch-1

license clarification + formatting all over