Add serialization and deserialization support

[NyanKiyoshi]

This pull request is a suggest fix for #91. (fixes #91)

• It adds a value_to_string method to MeasurementField that is called whenever django attempts to serialize the value, instead of using str(obj). In frontend, str(obj) is used.
• It adds an attempt at parsing a str value when the deserializer invokes the to_python method.

[codecov-io]

Codecov Report

Merging #92 into master will increase coverage by 0.52%. The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage    88.8%   89.33%   +0.52%     
==========================================
  Files           4        4              
  Lines         134      150      +16     
==========================================
+ Hits          119      134      +15     
- Misses         15       16       +1

Impacted Files	Coverage Δ
django_measurement/models.py	94.66% <87.5%> (-0.25%)	:arrow_down:

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[codingjoe]

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