Refactor variable type system and improve readme

[kunyuan]

1. Refactor variable system. Most of the variables are subtyped from AbstractVector, so we can support the array interface.

2. Add more examples in tutorials, following the suggestion from the issue #34 and issue #35 .

[codecov-commenter]

Codecov Report

Merging #36 (65a6640) into master (8a5d58d) will decrease coverage by 8.61%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   85.52%   76.91%   -8.62%     
==========================================
  Files          15       16       +1     
  Lines        1285     1295      +10     
==========================================
- Hits         1099      996     -103     
- Misses        186      299     +113     

Impacted Files	Coverage Δ
src/distribution/distribution.jl	46.66% <46.66%> (ø)
src/distribution/variable.jl	73.55% <77.77%> (-8.31%)	:arrow_down:
src/configuration.jl	84.78% <88.88%> (-10.28%)	:arrow_down:
src/vegas/montecarlo.jl	75.40% <0.00%> (-14.76%)	:arrow_down:
src/utility/color.jl	12.50% <0.00%> (-12.50%)	:arrow_down:
src/vegas_mc/montecarlo.jl	82.60% <0.00%> (-11.60%)	:arrow_down:
src/vegas_mc/updates.jl	80.55% <0.00%> (-11.12%)	:arrow_down:
src/mcmc/montecarlo.jl	78.26% <0.00%> (-10.15%)	:arrow_down:
... and 6 more

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