Typo
Fixed opening of Files

Removed explicit open mode, so that auto backup/auto restart is used also
for DUMPPROJECTIONS, DUMPDERIVATIVES and DUMPFORCES. Regtests has been added
with backup files.

I also removed some unnecessary c_str().
Fixed bug in ABMD

When MIN keyword was not used, an array was set as empty and later
accessed.
Changed open() to auto backup version
PLUMED_MAXBACKUP environment variable

Allows to switch off auto backup or to change the maximum number of 
backups
Improved doc for compilation/installation

Fixes #7
Merge branch 'v2.0'