close #258
don't clear feasible solution if status is not Optimal
close and add test for #259
update Cbc install instructions
BitArray in IndexedVector
Fix abstract, add indvec perf test
return nothing from addelt!
Merge pull request #262 from JuliaOpt/newidx

BitArray in IndexedVector
fix indexing bugs in printing code.
fix regression in printing for index sets w/ strings.
pretty things up and fix test
stray character from copy/paste job
Update print.jl
store Exprs and indices that define index sets.
add model printing test.
improved ordering for dictionary keys
check for hashing differences in printing test
Merge pull request #267 from JuliaOpt/jumpdict-print

New JuMPDict printing style
prep for 0.6.1 release
typo fix