Removed custum usage message on parse error

When the user sends wrong command line arguments the default behavior from
`argparse` python standard module is executed.
Removed the duplicated code for parsing the configuration file

At several places in the code logic is implemented to read the configuration.

The solution is to remove this duplication by pooling the business logic into
a single function (eg `parse_rc`).

Finally it will be better to read the configuration only once.
Improved the steps clarity into the main

Steps on the main are a little unclear.

So adding some comments to explain what's happening.

In the future it should be better to avoid writing a lot of code in it.
Fix platform detection for 32 or 64 bit Cygwin

If we're running in Cygwin's python, check sys.maxsize to determine
the Cygwin platform.

If we're running in Windows python, the python platform may not match
the Cygwin platform, so read the PE header of cygwin1.dll to determine
the Cygwin platform.

References:
http://en.wikibooks.org/wiki/X86_Disassembly/Windows_Executable_Files#File_Format
https://github.com/file/file/blob/b3f55f53/magic/Magdir/msdos#L79
Fixed file location for test
bug #8 Fixed the platform detection for 32-Bits or 64-Bits (alquerci, nylen)

This PR was merged into the 1.1-dev branch.

Discussion
----------

| Q               | A
| --------------- | ---
| Bug fix?        | yes
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Tests pass?     | yes
| Related tickets | #26, #33
| License         | GNU GPLv3

- [x] Added a test for detecting the right architecture
- [x] Running test under Windows 64 bits platform

If running `cyg-apt` on Windows 64 bits and with a python 64 bits then It is not an obligation that the Cygwin platform is also on 64-bit. To avoid that indecision maybe we should add an `architecture` configuration field for the version 1.2.

However I was not successfully used `cyg-apt` outside Cygwin, I'll try try again. This is another issue but this issue need it. Moreover I cannot tests it on 64 bits architecture.

So for the moments we cannot say it works for 64 bits architecture without an efficient test suite.

From #26:
> This PR implements most of the functionality described in #8, except for an option to set the package architecture, which should not be needed.

Commits
-------

f709f82 Fixed file location for test
830368c Fix platform detection for 32 or 64 bit Cygwin
2529ef4 Improved the steps clarity into the main
7749d13 Removed the duplicated code for parsing the configuration file
cecbfef Removed custum usage message on parse error