Fix arduino compilation errors

[Zvezdin]

This fixes #16.

More precisely:

1. Removes the unused StringSumHelper constructor in string.h
2. Adds documentation on how to build with the Arduino IDE

It also fixes the following bugs

1. The buffer in istream_Stream::readuntil is too small for some WebDAV implementations
2. Fixes VObjectStream exception when the input, as a list of VObjects, isn't wrapped in BEGIN/END VCALENDAR. This is the case for some VTODO queries. Now it simply ends the stream if the input ends as well.