Bugfix in std.conv.parseEscape

parseEscape was not parsing the escapes:
\
0
'
"
?
unittests for parseEscape
Update unittest to take into account...

...Octal escapes and named character entities. These still don't work, but there was a test checking octal would throw, which is wrong.
Fix typo in comment
Merge pull request #1181 from monarchdodra/convEscape

Bugfix in std.conv.parseEscape