Root namespace for exceptions

Prepend backslash to all PHP built-in classes for those who namespace the library.
minor #116 Root namespace for exceptions (vlakarados)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Root namespace for exceptions

Prepend `\` (backslash) to all PHP built-in classes for those who namespace the library.

This is quite necessary as while developing it's possible not stumble upon exceptions and this will kill production with an uncaught exception, which has a great chance because this library is mainly used in the core of the application where `try/catch` is used rarely.

Commits
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fac4177 Root namespace for exceptions
bumped version to 2.1
add service provider interface
updated CHANGELOG
moved lib/ to src/
removed the bootstrap file for tests
updated docs for 2.1