General string handling:

added substr()

Support for Shift-JIS:
added euc_substr()
added helper euc_char2byte_pos()

Some bug fixes in the string handling methods.

Moved mbstring library calls from utf8_* anf euc_* to general methods to reduce class size.


git-svn-id: https://svn.typo3.org/TYPO3v4/Core/trunk@192 709f56b5-9817-0410-a4d7-c38de5d9e867