Make sequence_equal compatible with order_by

order_by's sequence implementation did not have
const begin()/end() methods which broke sequence_equal
and probably order things too.

Add alias for const_iterator

Older generic code (e.g. before auto) might require
const_iterator, so provide it as an alias to iterator.

Fix dangling reference issues

Some operators created iterators, used them to return
a reference then destroyed the iterator. This does not
mesh well with sequences whose iterators return
references to data in the iterator, like those returned
by select(). Fixed by returning value types instead.

Take out enumerable::const_iterator

Having iterator and const_iterator in enumerable did
not make much sense since enumerable is in itself a
front for a sequence. It would've been trivial to copy
the enumerable to access the non-const methods.
So now enumerable only has an iterator and begin
and end are both const methods. If you want const,
use enumerable<const foo> (added a new as_const
method to simplify this).

Added tests for non-const sequences

All operators that support returning sequences of
non-const references now have tests for that.

Added cast operator

Update README.md

Essentially updating it post-merge of ubuntu-14-04 branch.