Fixed issue trying to set CMAKE_INSTALL_PREFIX.

The only way to set the install directory for ADIOS was by creating
an INSTALL_PREFIX environment variable. The CMake variable would get cleared out
if that value wasn't set. This change checks first to see if CMAKE_INSTALL_PREFIX
is set before trying to get the environment variable value to set as
CMAKE_INSTALL_PREFIX.
Merge pull request #29 from acbauer/cmake_install_fix

Fixed issue trying to set CMAKE_INSTALL_PREFIX.
Added adios_free_transports() to free up duplicate strings (method names) in adios_finalize()
avoid double init of transform spec (double allocation) when copying a variable definition
Free arrays in selections created (copied) by read_bp.c functions
free expiring groupinfo on advance step to avoid memory leak
selections.c test frees all app data at the end to let valgrind focus on adios memory leaks
Merge branch 'master' into adios-query