Add __init__ file and fix arty pullup constraints

[rohitk-singh]

Without init.py file, setup.py doesn't install the package

[rohitk-singh]

Fixed pullup contstraints in arty example design, else Vivado throws out critical warnings like this:

Parsing XDC File [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc]
CRITICAL WARNING: [Common 17-163] Missing value for option 'objects', please type 'set_property -help' for usage info. [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc:17]
CRITICAL WARNING: [Common 17-163] Missing value for option 'objects', please type 'set_property -help' for usage info. [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc:22]
CRITICAL WARNING: [Common 17-163] Missing value for option 'objects', please type 'set_property -help' for usage info. [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc:27]
CRITICAL WARNING: [Common 17-163] Missing value for option 'objects', please type 'set_property -help' for usage info. [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc:32]
CRITICAL WARNING: [Common 17-163] Missing value for option 'objects', please type 'set_property -help' for usage info. [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc:37]
WARNING: [Vivado 12-1023] No nets matched for command 'get_nets -hier -filter ars_false_path==true'. [/home/rohit/lca2018/miniconf/litex-buildenv/third_party/litesdcard/example_designs/build/gateware/top.xdc:68]

[enjoy-digital]

Merged, thanks.