Improve build script

* Fix a problem that it couldn't build CoreTweet.Universal
* Change the description of WithPcl switch
Add .travis.yml
build on non-free environment?

inspired by OpenTween
Add the build status to README
to be free
Be freedom

this should work
FIx a mistake on .travis.yml

oops
Fix installation process of travis

monodoc sucks
Make travis's mono update its own certificates

https://c2.staticflickr.com/4/3283/2896636237_24d14b00bf.jpg
Merge pull request #66 from CoreTweet/test/travis

Use travis from now on
Merge pull request #63 from azyobuzin/fix/msbuild

Improve build script
Fix UNIX time parsing (value passed by JSON.NET is long)
DateTime optimization

There's two kinds of optimization in this commit.

*	Common UNIX epoch object to make efficient and easy to read.
*	Use `Ticks' rather than `Seconds' in floating point values.
Merge pull request #67 from a4lg/unix-time-fix

Fix UNIX time parsing (value passed by JSON.NET is long)
Install doxygen in travis to reduce build time