minor fixes: renaming and restructuring

[soumyaray]

• ltVariables —> construct_names

• mimic folders in R/ with filenames:

  • estimate_*.R
  • assess_*.R
  • compute_metrics.R (specific stats formula will be performed here)

• duplicate rho_a (evaluation.R) and rhoA (reliability.R)

• remove "SEMinR" title from README.md — the logo suffices as the title

• add TravisCI badge (build passing) to README.md

• should TravisCI badge link to Travis build page?

• change interaction item names from con1.con2 to con1*con2

[soumyaray]

@NicholasDanks please wait till #63 is merged to develop

[soumyaray]

@NicholasDanks added some points and made checklists out of them