More stable datatypes, writing nullvals, and type testing

[karimbahgat]

Added float datatype.
More fully implemented date type by returning and writing datetime.date objects, as well as a list for backwards compat, somewhat similar to #18.
Fixed old boolean reading bug.
Added writing of null values for all types.
Date and boolean fields have predecided size, so user shouldn't be able to change those internals, so autosets size and decimal.

No new doctest failures, only the ones that will be fixed with #68.

Also created some basic tests for all the datatypes (which also works and loads correctly in QGIS, though QGIS seems to treat boolean as string) but wanted to avoid cluttering the lib with additional files. So thought it would be better if someone could use the test code and implement it correctly according to existing pyshp test conventions. Here is the test code:

import shapefile
from datetime import date

# write

d = shapefile.Writer()
d.field("string", "C")
d.field("date", "D")
d.field("datelist", "D")
d.field("datemiss", "D")
d.field("num", "N")
d.field("nummiss", "N")
d.field("float", "F")
d.field("floatmiss", "F")
d.field("booltrue", "L")
d.field("boolfalse", "L")
d.field("boolunkn", "L")
d.field("boolmiss", "L")

writerow = ["hello",
             date(1998,1,30), [1998,1,30], None,
             45, None,
             89, None,
             True, False, "wrongbool", None
            ]

print writerow

d.record(*writerow)
d.null() 

d.save("testtypes.shp")

# read

d = shapefile.Reader("testtypes.shp")
readrow = d.record(0)

print readrow