Add StringScanner#pos= (#315)

Adds specs and implementation of #pos= for manually setting position for
scanner.
Restructure opal/ into separate corelib and stdlib dirs
Remove examples dir. Examples will be available on website
Clarify differences between core and stdlib in README
Add ERB support back into lib and stdlib

A simple ERB compiler is now bundled with the opal lib. The ERB runtime
code is now also in stdlib so it can be required ready for use.
Remove opal-template from stdlib. Now deprecated
Move rack config into spec/
Fix buggy code generation when method missing turned off (#308)

Fragments were not being used when processing call sexps, which caused
errors when joinging them at the end of code generation. This commit
fixes that bug.
ERB should generate files into Template namespace

An older commit changed this to the ERB namespace, but Template is more
template language independant.
Add Opal::Builder for building corelib as single file

Builder is a simple class to build the opal runtime into a single
string. This is useful as it allows opal to build its runtime without
the need of opal-sprockets. Builder just looks for all the require
triggers in the source and builds up the list of files to build
together.
Bump opal-sprockets dev dependency to 0.1.1