tests: Add support for PB and TB to parse_si_suffix
tests: Add more assertions

Add two new assertions which are useful in many tests:
* assert_equals
* assert_less_or_equal
tests: Add test_petabyte_file

We have introduced checking of appending data to the a 1 petabyte file.
We use sparse file so we do not need to allocate all space to perform this test.
In addition we also make sure if the sparse file is correct (returns zeros from
not allocated blocks). The last think we check is if the appended data does not
occupy more storage than one CHUNK_SIZE.

Closes #109
tests: Add valgrind support

Add support for running tests under valgrind and reporting
valgrind-detected errors.

Allow forcing valgrind on arbitrary tests by setting USE_VALGRIND=1.

Add new 'test_write_and_read_with_valgrind' test case, which runs
the usual 'test_write_and_read' under valgrind.
tests: Add possibility to use loop devices as disks

This commits adds a possibilty to run 6 chunkservers, each of them using
a separate 1 GB filesystem as it's hard drive. This makes it possible
to test some features like disk usage reporting, rebalancing, etc.
tests: Add a test of chunk rebalancing
tests: Fix creating image files in setup_machine.sh

There were some problems with the setup_machine.sh script
when trying to call losetup -d on some machines. This commit
modifies the script so that it no longer uses losetup to create
disk images.
tests: Fix rebalancing test

Sometimes tests couldn't pass because current configuration moved more than one
chunk at once. It caused the disks usage differences couldn't be accepted.
This commit fixes test installation configuration.
tests: Remove disk lock files during cleanup

Remove lock files created in disk directories by the chunkservers.
Add the mfsrestoremaster script

Create a script to automate starting spare master on metalogger machine.

The script:
- rebuilds metadata from metalogger changelogs
- switches IP address to that of the master machine
- starts mfsmaster
tests: Add test_metadata_recovery

Add new test case which checks filesystem's ability to recover metadata
from metalogger logs.