Allow library to be initialized with a reader

[elynnyap]

• Add an option to initialize the browscap library with a Reader, instead of strictly from an .ini file. (We needed this option as we used an embedded asset for browscap.ini via go-bindata.)
• Add test for initializing from Reader.
• Improve current tests, which rely on a particular order of execution in order to pass; namely, they assume that the library has been initialized first. This change ensures that the tests still pass when run individually.